Mobile networks such as 2G (GSM), 3G (UMTS) and LTE telecommunications networks have an active state of communication with their mobile terminals and an inactive/idle state of communication with their terminals. When in the active state, as the mobile terminals move between different cells of the network, the communication session is maintained by performing a “handover” operation between the cells. In the inactive/idle state, as a mobile terminal moves between different cells of the network the mobile terminal performs “cell reselection” to select the most appropriate cell on which to “camp” in order that the mobile terminal can be paged by the network when mobile terminating data is destined for that mobile terminal.
Conventionally, generally speaking, the mobile terminal or network determines whether a handover/cell reselection procedure should be triggered in dependence upon measurements of the radio signals of the cells in the region of the mobile terminal. A filter is applied to the signals (either by the network or by the mobile terminal) which calculates an average (mean) value of these signals over a particular time period. This filtered/average values of the cells are then compared with each other. In dependence upon these comparisons, cell reselection/handover related procedures are triggered. This cell reselection/handover process generally comprises taking radio signal measurements of neighbouring cells and comparing these to each other and to the radio signal of the current cell to determine which cell provides the best signal strength/quality. Handover/reselection to the best cell can then occur.
In a mobile network operating in accordance with the 3G (UMTS) Standards, a mobile terminal device (UE) has a so-called “RRC (Radio Resource Control) state” which depends on its state of activity. In the respective RRC states different functions for mobility are executed. These functions are described in technical specification 3GPP TS 25.304/25.331.
For 2G and 3G, a mobile terminal is in active communication when it has a CS (Circuit Switched) connection established.
In 2.5G, GPRS PS (Packet Switched), active communication can be defined as the GPRS Ready state. In 3G UMTS PS, active communication can be defined as the RRC connected mode state that is CELL-DCH.
In 3G UMTS PS, CELL/URA_PCH and CELL_FACH can be defined as inactive states. In GPRS, the Standby state can be regarded as an inactive state.
Either one or both of the CS and PS active communications may occur in the mobile terminal.
For a 3G mobile terminal, in the active mode (and in CELL/URA_PCH and CELL_FACH) the terminal is in the RRC connected mode. The RRC connected mode includes the following states:
CELL_DCH state is characterized by:                A dedicated physical channel is allocated to the UE in uplink and downlink.        The UE is known on cell level according to its current active set        Dedicated transport channels, downlink and uplink (TDD) shared transport channels and a combination of these transport channels can be used by the UE.        
CELL_FACH state is characterized by:                No dedicated physical channel is allocated to the UE.        The UE continuously monitors a FACH (forward access channel) in the downlink.        The UE is assigned a default common or shared transport channel in the uplink (e.g. RACH) that it can use anytime according to the access procedure for that transport channel.        The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update.        In TDD mode, one or several USCH or DSCH transport channels may have been established.        
CELL_PCH state is characterized by:                No dedicated physical channel is allocated to the UE. The UE selects a PCH (paging channel) with the algorithm, and uses DRX for monitoring the selected PCH via an associated PCH.        No uplink activity is possible.        The position of the UE is known by UTRAN on cell level according to the cell where the UE last made a cell update in CELL_FACH state.        
URA_PCH state is characterized by:                No dedicated channel is allocated to the UE. The UE selects a PCH, and uses DRX for monitoring the selected PCH via an associated PCH.        No uplink activity is possible.        The location of the UE is known on UTRAN routing area level according to the URA assigned to the UE during the last URA update in CELL-FACH state.        
In the CELL_DCH state a network-driven handover is performed when necessary, as described in 3GPP TS 25-331. In this state a mobile terminal scans the pilot channels of up to 32 intra-frequency cells neighbouring its current cell. The mobile terminal forms a list of the best cells for possible handover based on the received signal strength and/or quality (i.e. the error rate in the received signal). The information in this list is passed to the UTRAN RNC on an event-driven basis, e.g. when the signal strength or signal-to-noise ratio of one of the cells exceeds a threshold. The information list is used by a handover algorithm implemented in the UTRAN RNC. The algorithm that determines when handover occurs is not specified in the GSM or UMTS Standards. The algorithms essentially trigger a handover when the mobile terminal provides a measurement of a neighbour cell received signal at the mobile terminal below a predetermined quality received threshold, which typically has a relation to the quality of the received signal from the serving cell (e.g. better quality by some margin).
In the “CELL_FACH”, “CELL_PCH”, “URA_PCH” or “idle mode” the mobile terminal may control its own mobility independently and starts a cell switch (reselection) when a neighbouring cell has a better quality than the current cell, as described in 3GPP TS 25.304. A similar procedure is also used in GSM/GPRS mobile networks, as described in technical specification 3GPP TS 05.08 (UE-based cell reselection).
In general, a mobile terminal in “idle mode” states and in RRC connected mode (inactive) states “CELL_FACH”, “CELL_PCH” and “URA_PCH” performs periodic measurements of its own as well as of a series of neighbouring cells. Information from the neighbouring cells is broadcast in the system information block 11 (SIB11) or system information block 12 (SIB12) of the broadcast channel (BCH) as described in 3GPP TS 25.304 and 3GPP TS 25.331.
In order to avoid a cell switch based on short-term changes in the radio field conditions, so-called “fading”, and the subsequent return to the original cell, a UMTS system mainly uses two parameters that are emitted in the Broadcast Channel (BCH) in the system information block 3 (SIB3) or system information block 4 (SIB4). Notably, these are the time interval “Treselection” and the hysteresis value “Qhyst”. In order to avoid too fast a switch between cells based on quickly changing network conditions, a switch from the original (“serving”) cell to the neighbouring (“target”) cell only takes place if the neighbouring cell was better than the original cell by the factor “Qhyst” for the time “Treselection”. The quality of the cells may be determined by measuring the Reference Signal Received Power (RSRP). This behaviour of a mobile end device is described in detail on the technical specification 3GPP TS 25.304. This can be expressed as:—If RSRP(target cell)>RSRP(serving cell)+Qhyst,RSRP THEN SELECT target cell
Multiple frequency layers and mobility state determination are provided in a similar manner for LTE/SAE networks.
Conventional access to the features and services provided by GSM and UMTS networks involves signalling between the mobile terminal and a standard base station (macro base station) that has a dedicated connection to an MSC and provides coverage in the cell occupied by the mobile terminal using cellular telecommunication (e.g. GSM or UMTS) transport protocols. There have recently been proposals to allow access to the features and services provided by GSM and UMTS networks by providing additional special base stations (femto base stations), referred to as access points (APs), for example at a subscriber's home or office, in order to increase network capacity and improve coverage. These access points communicate with the core network via IP based communications, such as a broadband IP network, and are typically routed via the Internet.
Many different names have been given to APs, such as home access points (HAPs), micro-base stations, pico-base stations, pico-cells and femto-cells, but all names refer to the same apparatus. APs provide short range, localized coverage, and are typically purchased by a subscriber to be installed in their house or business premises.
It has also been proposed to use APs in the LTE telecommunications network currently being developed, but not yet implemented.
An advantage of using an access point connected to the core network via an IP network is that existing broadband Digital Subscriber Line (DSL) connections can be used to link mobile terminals with the network core without using the capacity of the radio access network or transmission network of a mobile telecommunications network. In other words, the AP is integrated into a DSL modem/router and uses DSL to backhaul the traffic to the communication network.
A further advantage is that APs are able to provide mobile network access to areas where there is no radio access network coverage. Thus, they are expected to be particularly beneficial when installed in buildings with poor radio network coverage from the macro network but which have DSL connections. Additionally, an AP could provide UMTS coverage where there is no 3G coverage at all, perhaps only GSM coverage.
Handover and cell reselection are performed in the same way for APs. It is desirable for mobile terminals to provide continuous service when moving within an SAE/LTE coverage area and between an SAE/LTE and a UMTS coverage area/2G coverage area, and to/from APs.
Many mobile network systems in accordance with the UMTS standard are designed such that they use several frequencies and the development of the cells occurs in small cells (“micro-cells”) and larger cells (“macro-cells”). In general, this type of arrangement is called “hierarchical cell structure” (HCS) in cellular networks. This arrangement is described in 3GPP TS 25.304.
In HCS slow-moving or stationary mobile terminals should be located in the smallest possible cells, such as micro-cells, while (faster) moving mobile terminals are preferably located in larger cells, such as macro-cells. This reduces the number of cell switches for faster moving mobile terminals. In order to identify whether a mobile terminal is moving or stationary, HCS uses the determination of the number of cell changes (parameter NCR) over a specified period of time (parameter TCR) as described, for example, in WO-A-2001043462.
Both parameters NCR and TCR are reported to the mobile terminal via the BCH (in SIB3 or 4) in each cell and the mobile terminal decides using the number of cell changes (NCR) in time period (TCR) whether it is in a so-called “low-mobility” or “high-mobility” state. If the mobile terminal is in a “low-mobility” state, it favours a cell change in smaller cells (micro-cells) and in a “high-mobility” state, it favours larger cells (macro-cells). The result of this behaviour is that the number of cell changes for fast-moving mobile end devices is minimised, whereby the capacity of the mobile network is maximised overall.
Each time a device changes cell it is required to read all the system information transmitted on the cell etc